The present invention relates generally to the measurement of the current of an electrical load and more particularly to the production of a signal proportional to the average current of an inductive load supplied intermittently with electrical power when the load is provided with a free-wheeling current path.
It is often desirable to measure or monitor the average current of a load in order to provide a feedback signal which may be utilized to provide some control function. For example, adjustable speed d.c. motor drives are often required to operate in a constant horsepower mode. In this mode, the field is normally maintained at some nominal maximum strength until the rated motor armature voltage is reached. An increase of speed results in an increasing armature voltage. When a point called "crossover" is reached, the strength of the motor field is reduced to further increase speed. Crossover may be a level of armature terminal voltage or counter emf (CEMF). If CEMF is used, a signal proportional to armature voltage is summed with a signal proportional to armature current (with proper polarities) to derive a signal proportional to CEMF.
In systems of the type generally described above, one common practice was to use a rectifier circuit employing one or more thyristors to control the field current. Below crossover, the current would be held essentially full on and the field voltage would be a constant times the a.c. input voltage. As such, the field current will vary inversely as the field resistance and directly as the a.c. input voltage. In this situation, if the a.c. input voltage were high and the motor field were cold, the field current could be excessively large and the field could be excessively strong. To increase speed to some level above crossover requires a greater field current change than if the field current started from a more modest level. This is due to the presence of saturation. Since the field circuit is highly inductive and time is required to decrease the field current, the dynamic performance of the system in this situation tends to be degraded.
An additional feature often included in many prior art systems is a minimum field adjustment utilized to limit the field current undershoots and, therefore, speed overshoots. This adjustment normally involves a minimum to which the thyristors can be adjusted in phase retardation. Again, the actual field strength is directly proportional to the a.c. input voltage and inversely proportional to the field resistance. Here, if the drive system is adjusted while the field is hot with a minimum field setting very near to the actual current required at top speed, it might be impossible to reach the top speed when the field is cold.
From the above discussion the desirability of being able to provide an accurate feedback signal proportional to the current through a load such as a motor field is apparent. In many instances the provision of such a feedback signal presents no problem. For example, in the case of a d.c. motor supplied from a diode rectification bridge it is a relatively simple matter, utilizing a transformer, to measure the current on the a.c. side of the bridge and to assume, with accuracy, that the d.c. current on the load side of the bridge will be proportional to the a.c. current. There are, however, many instances in which power is not continuously applied to the load but is supplied thereto intermittently. This is particularly true in the control of motors. In motor controls it is quite common to employ a system utilizing switches, such as thyristors, to connect the motor to a power source such as an a.c. line or a battery. The percent of on time to off time governs the amount of power supplied to the motor. In many of these applications the motor is provided with what is commonly called a free-wheeling path which path provides a circuit through which the load current may flow during periods when the load is not connected to the source. Because load current will exist during periods when the load is not connected to the source, it is apparent that a measurement on the a.c. side of the switching devices will not provide an accurate representation of the average load current.
Direct measurement of the d.c. current has not heretofore been practical or has been very expensive. Devices such as Hall Effect devices in series with the load are known but are expensive and difficult to maintain with accuracy. A sensing component such as a resistor used in conjunction with a differential input amplifier is normally not satisfactory since it is not feasible to have enough voltage drop in the resistor in comparison to the common mode voltage. That is, the normal voltage across the load is very large compared to deviations therein and the use of a dropping resistor of a resistance great enough to develop a significant voltage results in wasted power and associated heat problems. The use of a conventional current transformer in a system employing a free-wheeling path in the a.c. side has been discussed and the use of a similar transformer in series with the load is not satisfactory inasmuch as the current through the load is primarily d.c. with a very low ripple content and the transformer is incapable of transforming the d.c. component of the load current.